Flat panel displays have become commonplace and are used in nearly every electronic device today. The need to process and provide information to the user in a compact and highly reliable manner has driven most of the research and development in flat panel displays. A wide variety of display technologies exist, many of which originated between 1960 and 1980. Some commercially important flat panel displays are the light emitting diode (LED) display, plasma display (PDP), electroluminescent (EL) display, vacuum fluorescent display (VFD), and liquid crystal display (LCD). While the LED enjoyed early success, the LCD has overtaken the LED as the device of choice in nearly all electronic apparatus, particularly in communication devices.
Dot matrix LCD's utilize a technique known as active matrix addressing or direct addressing. Rather than multiplexing the timing of the signals to select and write to particular lines of the display, the addressing function is separated from the writing function, thus each line can be written more quickly. As more and more lines are written, the amount of time the controller or display driver can spend writing to each individual line or pixel (the duty cycle) decreases. Eventually, the molecules of the liquid crystal do not have time to react to fully react to the applied voltage, and image quality diminishes. Large dot matrix displays that are not actively addressed exhibit slow speed, high power consumption, and complex circuitry. The technique of active matrix addressing makes the display hardware more complex by adding a switch to each pixel. The switch can then be turned on very rapidly (in a few microseconds) and a storage capacitor can then be used to maintain its condition while the other lines are being written. Several approaches to making individual switches have been investigated, and these include diodes, varistors, transistors and various combinations thereof. The thin film transistor (TFT) approach has emerged as the most successful technique for active matrix addressing in terms of display performance. This structure utilizes a thin film transistor and capacitor applied directly to the display substrate. Obviously this places severe requirements on both the hardware and processing conditions needed to fabricate the display. A semiconductor material must be incorporated into the substrate in order to fabricate the thin film transistor, with polysilicon or amorphous silicon the two techniques of choice to create the semiconductor material for the transistor. Since this approach essentially requires that the transistor be located in close proximity to each pixel, the substrate for the display must be fabricated in the same manner as semiconductor wafers, thus incorporating significant cost and yield penalties in making the substrates. While displays used for small screen televisions that typically measure 3 to 6 inches (diagonal) can be made using this technology, clearly anything above 10 inches becomes cost-prohibitive or impossible to fabricate.
Another type of display, the EL, utilizes a mechanical structure similar to the LCD, but instead of using a liquid crystal fluid as the image forming medium, an electroluminescent material is substituted. When the voltage is applied to the EL material, it is energized and then decays to emit a photon and thus creates light in the visible spectrum. Typical EL displays utilize an inorganic electroluminescent material such as zinc sulfide that is doped with manganese or other ions. Examples of these types of EL displays may be found in U.S. Pat. No. 4,924,144 which is incorporated herein by reference. EL displays suffer from the same limitations that LCD's do; namely the need to provide high speed accessing of the individual pixels in large dot matrix applications. While EL panels can be switched at a higher rate than LCD's, thus making them more attractive for large flat screen displays, the inherent problems of making large displays still remain. Clearly, it would be desirable and a significant step in the art if a technique to make a cost effective, high speed, large, flat panel display could be provided.